Photoresists in Lithography
Lithography, in general terms, is the process of printing or transferring a pattern onto a surface. Optical lithography plays an important role in the production of semiconductor devices, among other things. In semiconductor lithography patterns of conductors and insulators are written onto a semiconductor substrate. One technologically important use of semiconductor lithography is in the production of an integrated circuit (IC). In an IC, lithography is typically used to write patterns of insulators, conductors, doped regions, etc. onto a semiconductor substrate, usually a silicon wafer. These patterns, once written, are then used to control the electric fields and electric currents within the IC. Advances in semiconductor lithography are important for the industry since lithography often tends to be the technical limiter for further advances in transistor size reduction.
A simplified view of a generic optical lithographic process is as follows. Photoresist material is deposited onto the substrate or an intermediate layer which is to be patterned by means of a deposition process, such as, but not limited to spin casting, drop-casting, or spray-casting. Photoresist (or resist), is a polymeric material which undergoes a chemical reaction, or series of chemical reactions, upon exposure to light. Then light from a light source is passed through a photomask, which lies between the light source and said substrate and serves to introduce a pattern into the light, also known as the aerial image. The patterned light impinges upon the photoresist layer where it exposes the resist. There are a variety of methods for exposing the resist including, but not limited to contact lithography, proximity lithography, projection lithography and immersion lithography. Upon exposure, the exposed area of the resist undergoes a chemical change, creating what is known as a latent image in the resist. This light induced chemical change usually leads, either directly or indirectly, to a change in the solubility of the resist. The resist is then developed, a step which translates the chemical changes in the resist into a change in the coverage of the resist on the substrate by removing selected areas of the resist. The physical pattern in the resist is then transferred to the underlying substrate by various means, including, but not limited to: etching of the substrate, deposition of conductors or semiconductors, and ion implantation. Methods of etching the substrate include, but are not limited to: wet etch, plasma etch, and reactive ion etch (RIE). In this manner a pattern can be transferred onto the semiconductor substrate. In order to create more complex structures on the semiconductor substrate, the light exposure and physical pattern transfer steps (deposition/etch/ion implantation/etc.) may be repeated a number of times, with the same or different light mask patterns, with each subsequent pattern being aligned with the previously transferred pattern on the wafer.
The light driven chemical reactions which occur in the photoresist are most often utilized to selectively change the solubility of the light exposed regions of the polymer, allowing the selective removal during later processing. The ranges of the electromagnetic spectrum from the light source which are used to expose the resist in optical lithography, may be chosen from a wide variety of ranges, including, but not limited to: visible light, ultraviolet (UV) light, extreme ultraviolet (EUV) light, electron beams, ion beams and x-rays. The light sources may be relatively more monochromatic or polychromatic in nature. Also within these different light ranges, a variety of wavelengths may be chosen. Wavelengths for optical lithography include, but are not limited to: 435 nm, 365 nm, 248 nm, 193 nm, and 157 nm. For the case of positive resists, the resist become more soluble in developer upon exposure, and thus the exposed areas may be selectively removed during development. The opposite is true for negative resists, where the resist becomes less soluble upon exposure. A wide variety of different chemical mechanisms have been exploited for both positive and negative. Two broad classes of resists are the conventional resists and the chemically amplified resists. In both classes of resist, the structure and etch resistance of the films are provided by the polymeric resin.
In conventional resists, mainly exposed with 436 nm and 365 nm light, the absorption of light by a photoactive compound (PAC) leads directly to a change in the solubility of the resist. Common examples of conventional resists include the diazonaphthoquinone (DNQ)/novolac family of resists. These positive resists are composed of DNQ, a PAC, mixed with a novolac resin. DNQ inhibits the dissolution of the novolac resin by the developer solution. Upon absorption of a photon the DNQ is converted to a carboxylic acid. The presence of the carboxylic acid acts to increase the solubility of the resin in the developer, leading to the efficient removal of the exposed areas of the resist during development.
In chemically amplified resists, by way of contrast with conventional resists, the product of the photochemical reaction does not appreciably change the solubility of the resist resin directly. Instead, in a chemically amplified resist the light is absorbed by a chemical species which as a result of the absorption process generates another active chemical species which then in turn can catalyze reactions in the resin to change the resin's solubility. One group of such photochemical generators, which are widely utilized in chemically amplified resists are photoacid generators (PAGs). As a result of the absorption of a photon the PAG generates an acid. During a post exposure bake (PEB), the resin then undergoes thermally induced reactions catalyzed by the photogenerated acid, resulting in a change in solubility for the resist. One non-limiting example of a thermally induced reaction which may take place in a positive tone chemically amplified resist, is the acid catalyzed deblocking of the resin. The deblocked resin can be much more soluble in the developer solution and thus be removed. A non-limiting example of a thermally induced reaction which may be exploited for a negative tone resist, is the acid catalyzed activation of a cross linking agent. The cross-linked, and thus effectively higher molecular weight resin, may be significantly less soluble in the developer solution and thus selectively remain behind during the development stage. In addition to photoacid generators, there are also other photochemical generators, such as photobase generators and photoradical generators, which generate bases and radicals respectively upon the absorption of light, which may be utilized in chemically amplified resists. Recently there has been exploration of negative tone development. In negative tone development, a traditional chemically amplified resist, which are currently being used in leading edge lithography production as positive resists, is used but the developer is replaced with an organic solvent that dissolves the blocked portion of the resin.
The smallest feature size that can be printed by an optical lithography system, also referred to as the resolution of the system, is determined by two main factors. These are the smallest image features that can be projected onto the substrate and the ability of the photoresist to resolve the image and translate that image into a latent chemical image. The first factor is mainly determined by the feature size of the mask, the wavelength of light used to create the aerial image and the numerical aperture, in the case of projection optical lithography. The resolving power of the photoresist depends on the type of resist, i.e. conventional or chemically amplified, and the specifics of the resist. In chemically amplified resists there is a relationship between the resolution, line-edge roughness, and sensitivity of the photoresist, known as the RLS triangle. In simple terms it states that only two of the three properties can be optimized in the formulation of the resist. For example, a high resolution resist which can produce features with low line-edge roughness, will, according to this relationship suffer from low sensitivity. The ability to produce a photoresist which can combine all three desirable properties may enable new technologies.
Block Copolymers
Polymers are macromolecules which are composed of smaller repeating structural units, often referred to as monomers, chemically bound to each other. Homopolymers are polymers in which the constituent monomers are chemically identical. Copolymers are a class of polymers which are composed of at least two different monomeric species. Block copolymers are copolymers in which the different monomeric species are segregated into homopolymer subunits or blocks which are covalently bonded to each other. At the juncture of the homopolymer blocks there may also be an intermediate subunit added for the sake of chemical compatibility. In conceptual terms then, a block copolymer can be thought of as two or more discrete homopolymers linked covalently end to end. Block copolymers composed of two and three homopolymer blocks are known as diblock copolymers and triblock copolymers, respectively. Block copolymers with even more blocks are also known.
The different blocks, may have properties which make it energetically unfavorable for them to mix with each other at a molecular scale, thus resulting in the phase separation of the different blocks. One example may be the case of amphiphilic block copolymers, where one block is lipophilic and the other block is hydrophilic. The resulting copolymer chain therefore is a single molecule which will mix with other copolymer chains in a way to create a polymeric material which can have chemically distinct periodic domains created by the phase separation of the different polymeric blocks. Bulk or macroscopic phase separation cannot result because the blocks are bound to each other in a single polymer chain. The result is that the blocks within the block copolymer self-assemble into repeating patterns (or arrays) of small domains.
The domains created by the phase separation of the block copolymers can be made to have a variety of sizes and shapes and geometries depending upon several factors but mostly due to the relative lengths of the homopolymer blocks. The sizes of the block copolymer domains may vary a good deal depending upon the exact makeup of the copolymer, but sizes usually vary in the range of 5 to 50 nm. The natural period of the block copolymer is often used when describing the size of the domains in the block copolymer. The natural period, L0, is the defined as twice the length of the block copolymer in its ordered state (H. Kim, W. D. Hinsberg, J. Vac. Sci. Technol. A 22 (6), 2008, 1369). For the case of an ideal diblock copolymer the geometry or morphology of the resulting domains changes as a function of the relative chain lengths of the homopolymer blocks and thus by extension the volume fraction of a given block. For example, for a diblock copolymer with a low volume fraction of one block, the minority blocks form spherical domains arrayed within the bulk of the majority polymer. At higher fractional volumes of one block, the minority blocks form cylindrical domains which are arrayed in a hexagonal fashion within the bulk of the majority polymeric block. For an ideal diblock copolymer where the relative amount of one block is about the same as the other domain, the two blocks create parallel lamellar domains.
The thickness of a block copolymer film can also strongly influence the phase separated block domain structure or morphology. This is mainly due to the way in which changing the thickness of the film changes the relative important of the interactions of the copolymer at the block copolymer/air and block copolymer/substrate interface. For films thinner than the natural period of the block copolymer, L0, the formation of equilibrium type domains may be frustrated by the increased importance of the block copolymer interfaces. This usually results in the formation of complex morphologies. For the formation of patterns which are more typically of interest to lithography, the desired film thickness is on the order of the natural period of the block copolymer, L0. In this range the effect of the interfaces usually has more influences over the long range ordering or directionality of the domains rather than the shape of the domains. The domain shapes and sizes for films in this thickness range are typically closer to those for bulk block copolymers where the effects of the interfaces are minimized.
Deposition of block copolymer films can be performed by a variety of methods, including, but not limited to drop-casting, spin-casting, dip-coating, or spray-casting. After being deposited, the block copolymers are usually not in an equilibrium or near-equilibrium state, i.e. they have not formed the morphology of the phase separated domains which would minimize the energy of the film. An anneal step, or series of annealing steps, are then used to drive the morphology in the direction of equilibrium or near equilibrium structures and create a film with the desired self-assembled domains made up of the blocks of the copolymer. These thermal annealing step usually raise the temperature of the block copolymers above the glass transition temperatures of the constituent blocks.
Lithographic Patterning with Block Copolymers
For years the ability of block copolymers to create patterns on the nanoscale has been well known. There have already been examples of copolymer thin films being used for in the creation of etching masks for the transference of patterns to a substrate as well as further promise of creating patterns where the domain sizes are in the range of interest for current and future lithographic features. Early work demonstrating the use of the phase separated domains in block copolymers to pattern a substrate date back to over 20 years. Thin films of block copolymers may be formed on a substrate and various processing methods, such as selectively etching one block of the copolymer, may then be used to transfer the pattern to the substrate. The resulting patterns may be smaller than what can be easily achieved by purely optical methods. The patterning of block copolymers can potentially supplement optical lithography in making certain patterns with feature sizes that are comparable to or smaller than what is achievable with more conventional means and often in a simpler or cheaper manner.
While the range of domain geometries fall loosely within the three listed above (lamellar, cylindrical, and spherical) these geometries can be used to create at least two important lithographic patterns. For example, thin films of block copolymers with spherical domains or cylindrical domains orientated perpendicular to the substrate may be used to make via or point contact patterns. In a similar fashion, both lamellar and cylindrical domains orientated parallel to the substrate may be used to create line-space patterns.
Since the size of the pattern features given by block copolymer domains and their sharpness directly relate to the size and chemistry of the blocks which make up the copolymer, the control of the dimensions of the repeating domains and their short range ordering can be quite robust. However, the use of block copolymers to generate lithographic patterns can often be limited by a relative lack of longer range order. Long range order is defined as length scales of several natural periods of the block copolymer, compared to localized or short range order, which would be in the range of one to two natural periods. For example, if a line-space pattern with equal-lined, equal-spaced distances is desired, a block copolymer may be composed and spin cast onto a substrate in such a way as to create a lamellar structure with lines and spaces determined by the respective and absolute sizes of the various blocks, among other factors. The pattern created by the block copolymer may have very good short term order, i.e. the dimension of the domains which will make up the spaces and the lines may be as desired and the lines and spaces may be parallel, but simply drop casting the film of block copolymers may not create long range order, i.e. with all the lines and spaces running straight from one end of the substrate to the other and oriented in the correct manner.
There are many ways to maximize, or at least to increase, the long range order of the block copolymers, including, but not limited to: increased film annealing times in the presence of solvent vapor, directional solvent vapor evaporation techniques, the introduction of shear forces, the use of a temperature gradient, and directed self-assembly. From a lithographic standpoint, the most useful methods for introducing more long range control over the alignment and orientation of the domains within the block copolymer are the methods of directed self-assembly. In these methods the long range order is transferred to the block copolymer from patterns previously written into the substrate. The use of these methods may often require the use of complementary lithographic techniques. Two of these methods to help introduce long term order into patterns of are known as chemical epitaxy and graphoepitaxy. In both of these methods, the substrate or a thin film deposited on the substrate has a background pattern already written into it. The features of this pattern are usually larger than the desired feature size for the block copolymer and the main purpose is to create a long term ordering which the self-assembled pattern of the blocks of the copolymer can build upon. One example of this might be to use optical lithography to create a line and space pattern in a resist, hardmask, or substrate, with a relatively large feature size and then use the self-assembly of the block copolymers to perform a pitch division. Deposition of the block copolymer, previously optimized to create a lamellar type geometry, onto the coarsely patterned surface may then, with the aid of further processing, write finer lines and spaces aligned within these coarse features. In this example the coarse features written by the optical lithography would define the directionality and alignment of the finer domain features. In graphoepitaxy the chemistry of the walls and floors of the spaces can strongly effect the ordering of the phase separated block domains. For example, if features that are perpendicular to the substrate are desired, often the case for efficient pattern transfer of both lamellar and column type features, the “floor” of the space (substrate, hardmask, underlayer, etc.) should be neutral with respect to wetting by the two blocks of the block copolymer. If instead one block preferentially wets the surface, the pattern will tend to have components which may vary with the depth of the feature, which is significantly less desirable. The chemistry of the walls of the space can also play an important role in the final morphology and long range ordering of the block copolymer film.
For the case of chemical epitaxy, chemical differentiation on a flat surface is used to direct the self-assembly process, inducing long range order. These sparse patterns of chemical differentiation can be written by various techniques involving a variety of lithographic technologies, including optical and e-beam lithography. The chemistry which interacts with the blocks of the copolymer and directs the self-assembly are usually of a pinning or preferential wetting type. Features written with chemical differentiation potentially have better resolution (smaller line-edge roughness, etc.) than graphoepitaxy, but it is usually harder to write these chemical patterns.
Cleavable Block Copolymers
Recently there has been an interest in the development of cleavable block copolymer for a variety of uses including drug delivery and the production of nanoporous surfaces. Cleavable block copolymers are copolymers which can undergo a reaction to break the linkage between two of the blocks of the copolymer. This is usually achieved by placing a reactive chemical unit or linker between the neighboring blocks of a block copolymer. Under exposure to a specific stimulus the linker undergoes a chemical reaction, the end result of which being the breaking of the chemical bonds within the linker or between the linker unit and one or both of the blocks of the copolymer to which it is attached. Thus the block copolymer can be split into two separate segments along the juncture of two blocks. In the case of a block copolymer, the result would be to break the copolymer into two separate segments each containing only the one block and possibly fragments of the linker molecule. Several different linker groups with a variety of driving stimuli (photons, acid exposure, etc.) have been developed to this end. A non-exhaustive list of examples of photocleavable block copolymers (block copolymers which can be cleaved by exposure to light) include: block copolymers in which the linker is an ortho-nitrobenzyl group {Synthesis of Photocleavable Poly(styrene-block-ethylene oxide) and its Self-Assembly into Nanoporous Thin Films} or an anthracene photodimer {J. T. Goldbach, K. A. Lavery, J. Penelle, T. P. Russell Macromolecules 2004, 37, 9639.}, both of which can be cleaved by exposure to UV light. There are also examples of block copolymers which can be cleaved by exposure to an acid. Examples of this include block copolymers with a triphenylmethyl (trityl) ether linkage {M. Zhang, L. yang, S. Yurt, M. J. Misner, J. Chen, E. B. Coughlin, D. Venkataraman, T. P. Russell, “Highly Ordered Nanopourous Thing Films from Cleavable Polystyrene-block-pol(ethylene oxide)” Adv. Mater. 2007, 19, 1571.}, a tert-butyl carbamate linker {M. A. DeWit, E. R. Gillies, “A cascade Biodegradable Polymer Based on Alternating Cyclization and Elimination Reactions” J. Am. Chem. Soc. 2009, 131, 18327.}, a diphenyl methyl ether linkage {S. K. Varshney, J. Zhang, J. Ahmend, Z. Song, V. Klep, I. Luzinov, “Synthesis of poly(styrene-block-ethylene oxide) copolymers by anionic polymerization and acid cleavage into its constituent homopolymers for the formation of ordered nanoporous thing films” e-Polymers 2008, 94.}